1. Field of the Invention
The present invention relates to a heat-conductive silicone rubber composition which displays superior handling and moldability, and imparts a good level of adhesion, and also relates to a highly heat-conductive silicone rubber molded product produced by curing such a heat-conductive silicone rubber composition.
2. Description of the Prior Art
Heat generating components such as power transistors and thyristors suffer from a deterioration in performance as a result of the generated heat, and as a result, heat sinks are conventionally attached to these components during installation to enable the generated heat to diffuse and escape through the metallic chassis of the equipment. In this process, in order to improve the electrical insulation and the thermal conductivity, a heat radiating insulating sheet formed from a silicone rubber containing a heat-conductive filler is typically employed between the heat generating component and the heat sink.
Examples of this type of heat radiating insulating material include the insulating compositions disclosed in Japanese Laid-open publication (kokai) No. 47-32400 (JP47-32400A), comprising 100 parts by weight of a synthetic rubber such as silicone rubber, to which is added 100 to 800 parts by weight of at least one metal oxide selected from a group consisting of beryllium oxide, aluminum oxide, hydrated aluminum oxide, magnesium oxide and zinc oxide.
Furthermore, examples of heat radiating materials which can be used in those situations in which insulation is unnecessary include the compositions disclosed in Japanese Laid-open publication (kokai) No. 56-100849 (JP56-100849A) corresponding to U.S. Pat. No. 4,292,223, U.S. Pat. No. 4,292,224, U.S. Pat. No. 4,292,225 and U.S. Pat. No. 4,293,477, comprising an addition curing type silicone rubber to which is added 60 to 500 parts by weight of a heat-conductive powder such as silica, silver, gold or silicon.
However, these heat-conductive materials display a thermal conductivity of no more than 1.5 W/mK, and if a large quantity of a heat-conductive filler is then added via high filling to improve the thermal conductivity, the fluidity decreases, causing a significant deterioration in the molding workability of the composition.
In an attempt to resolve this problem, Japanese Laid-open publication (kokai) No. 1-69661 (JP1-69661A) discloses highly heat-conductive rubber and plastic compositions filled with an alumina filler comprising 10 to 30% by weight of alumina particles with an average particle diameter of no more than 5 μm, and the remainder of spherical corundum particles with no cutting edges and with an average particle diameter for a single particle of at least 10 μm. Furthermore, Japanese Laid-open publication (kokai) No. 4-328163 (JP4-328163A) corresponding to U.S. Pat. No. 5,352,731, discloses heat-conductive silicone rubber compositions comprising a base formed from a combination of a gum type organopolysiloxane with an average degree of polymerization of 6,000 to 12,000, and an oil type organopolysiloxane with an average degree of polymerization of 200 to 2,000, to which a spherical aluminum oxide powder is added in a ratio of 500 to 1,200 parts by weight of the aluminum oxide powder per 100 parts by weight of the base polymer constituents.
However even using these methods, if high filling is performed using more than 1,000 parts by weight of aluminum oxide powder per 100 parts by weight of the base polymer constituent (aluminum oxide exceeding 70 volume %), then simply adjusting the particle combinations or altering the viscosity of the silicone base is unable to produce a satisfactory improvement in the molding workability.
Consequently, as a method of improving the molding workability, Japanese Laid-open publication No. 2000-256558 (JP2000-256558A) corresponding to U.S. Pat. No. 6,306,957, discloses heat-conductive silicone rubber compositions comprising 0.1 to 50 volume % of a hydrolyzable group containing methylpolysiloxane as a wetter. According to this method, although the molding workability of the heat-conductive silicone rubber composition is improved, a major problem arises in that the adhesion of the composition to a substrate is inadequate.